Left ventricular heat dissipation will be determined in intact dogs. In this method, a known quantity of heat is injected into the myocardium via the coronary arterial inflow and the percentage of this "excess heat" recovered in the coronary sinus is measured. This percentage figure must be the same for the fraction of the total left ventricular heat which is dissipated via the coronary sinus outflow. The total left ventricular heat can thus be derived from measuring coronary sinus heat dissipation. From measurements of heat dissipation and work, mechanical efficiency is obtained as work/work and heat. The magnitude and variability of left ventricular efficiency will be studied under the effects of inotropic interventions, coronary infarction, chronic heart disease, hypertension and hypertrophy. Anaerobic contributions to left ventricular energetics will also be quantitated. In addition, the effects of induced changes in aortic smooth muscle activity on the efficiency will be studied. Many cardiovascular control mechanisms such as, e.g., heart rate control, may subserve the maintenance of optimal, i.e., most efficient energy transfer from the heart to the arterial system. This investigation will be able to document whether or not some of the control mechanisms have this functional significance.